Resilient tires and wheels – Tires – resilient – Anti-skid devices
Reexamination Certificate
2000-03-10
2002-09-03
Maki, Steven D. (Department: 1733)
Resilient tires and wheels
Tires, resilient
Anti-skid devices
C152S209210, C152S901000, C152SDIG003
Reexamination Certificate
active
06443200
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a tread pattern for a tread intended for a tire for heavy vehicles; it also relates to a mold making it possible to obtain such a tread pattern by molding.
In order to maintain, or even improve, the performance of a tread pattern of a tire comprising elements in relief under conditions of travel on slippery ground (such as, for example, ground covered with water), it is known to make on said elements a plurality of cutouts in the form of grooves and/or incisions (an incision having a width less than the width of a groove, and generally the width of an incision is less than 3 mm). These cutouts in total cross the width of the rib to interconnect the grooves defining said ribs.
It is also known that the performance on slippery ground is improved when the number of cutouts is large; however, an excessively large number of cutouts results in a great loss of rigidity of the elements in relief, which results in a general loss of performance, even on non-slippery ground. In fact, this loss of rigidity causes movements of greater amplitude for the points of the tread pattern which are in contact with the ground, which causes, among other disadvantages, so-called irregular wear in that they are localized (in particular on the ridges of the ribs) and no longer uniformly distributed over the entire running surface of the tread. This irregular wear significantly reduces the useful life of the tire provided with said tread pattern.
It is also known that the distributions of contact pressure between each element in relief of a tread pattern and the ground have a direct influence on the regularity of wear of said tread. For example, in the case of a tread pattern provided with a plurality of ribs defined by grooves of substantially longitudinal orientation (corresponding to the circumferential direction on the tire), it is known to mold grooves which, when viewed in cross-section, have widths which increase as the tread becomes worn.
By definition, the walls of the elements in relief defined by such grooves are said to be undercut relative to the running surface of the tread.
This arrangement makes it possible to make the ribs flexible in the vicinity of their ridges (defined as being the intersections of the lateral walls of each rib with its wall surface intended to come into contact with the ground) and thus to reduce the contact pressures of each rib with the ground close to said ridges. This reduction in pressure is more particularly substantial when the tread is new, that is to say, when the thickness of the tread is at a maximum.
However, if said latter tread pattern is provided with a plurality of incisions to increase the performance of adhesion on slippery ground, then the combination of a low pressure in the vicinity of the ridges of the ribs with movements of great amplitude linked to the presence of said incisions results in very pronounced irregular wear of these ribs in the vicinity of their ridges.
One known method of avoiding this irregular use is to make a plurality of incisions opening solely on to the surface wall of the ribs without cutting into the lateral walls and the ridges of the ribs; however, this arrangement results in noise problems which are linked to air being trapped within said incisions when they pass into contact with the ground during travel.
SUMMARY OF THE INVENTION
One object of the invention is to propose a tread for a tire, the tread pattern of which comprises a plurality of elements in relief defined by at least two grooves, and at least two of the lateral walls of which are undercut, some of said elements in relief being provided with at least one incision opening on to the running surface of the tread when new, said tread not having the disadvantages which have been discussed above.
There is proposed a tread for a tire which is provided with a tread pattern comprising a plurality of elements in relief defined by grooves, each element in relief being provided with a surface wall forming part of the running surface and intended to come into contact with the ground during travel, and at least two lateral walls which are undercut relative to the surface wall, the intersection of each of the lateral walls with the surface wall forming a ridge.
The tread according to the invention is characterized in that:
at least one element in relief is provided with at least one incision opening on to the surface wall thereof and on to at least one of its lateral walls, and
in that the incision(s) do(es) not cut into the ridge common to said surface wall and to said lateral wall at least between a first depth H
1
and a second depth H
2
, H
2
being greater than Hi, these depths H
1
and H
2
being measured perpendicular to the surface wall of the element and corresponding to different levels of wear of the tread.
At least two of the lateral walls of one and the same element in relief are undercut, that is to say that they are inclined such that the width of the surface wall of said element decreases as the tread becomes worn.
The tread pattern according to the invention makes it possible to improve the performance of the tread pattern on ground which has been made slippery, owing to a number of incisions which may be large, without creating a problem of noise or of irregular wear on dry ground. Each element in relief of the tread pattern according to the invention may comprise a plurality of incisions which open both onto the contact surface of the element with the ground and into at least one of the grooves defining the element in relief to permit the evacuation to at least one of said grooves of the fluids entering the incision during travel.
It should be noted that as a general rule the maximum depth of the incisions is virtually equal to the depth of the grooves defining the elements in relief to benefit from a large number of ridges during the entire period of use of the tread; however, it is of course possible to provide different depths.
Preferably, the depth H
1
is low compared with the depth of the incision in question, so as to retain sufficient rigidity for the element in relief, in particular when said element is subjected to forces of contact with the ground which are directed parallel to the ground and in a direction perpendicular to the average orientation of the incision; “low” is understood to mean that H
1
is at most equal to 10% of the deepest of the cutouts (grooves and incisions included).
Each incision has on the running surface a geometric trace having an average orientation defined as being the angle formed with the longitudinal direction of the tread by the straight-line segment joining the points of said trace which are farthest from each other.
The tread pattern according to the invention has the advantage of perceptibly reducing the travelling noise on dry ground created by the presence of incisions due to the fact that these incisions open into at least one groove and can thus evacuate the air which they contain as they pass into contact with the ground.
This tread pattern, despite the large number of incisions, has the advantage of modifying relatively little the distribution of the contact pressures of each element on the ground, in particular close to the ridges, while alleviating the reduction in rigidity during the first part of use of the tread (that is to say, up to about a level of wear corresponding to half the thickness of said tread).
A good compromise between the maintenance of sufficient rigidity for an element in relief and optimum evacuation of the fluids contained in the incisions of said element in relief towards the adjacent grooves is achieved once the depth H
2
is between 40% and 90% of the depth of said incisions.
Preferably, the incisions which open both on to the surface wall and on to at least one lateral wall of an element in relief do not cut into any of the ridges of said element when new (in this case: H
1
=0); the object of this arrangement is not to reduce the rigidity of the tread in the vicinity of the surface wall when new.
One particularly a
Baker & Botts LLP
Maki Steven D.
Michelin & Recherche et Technique S.A.
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